Layer detection for high aspect ratio etch control

ABSTRACT

Controlling an etch process applied to a multi-layered structure, by calculating a spectral derivative of reflectance of an illuminated region of interest of a multi-layered structure during an etch process applied to the multi-layered structure, identifying in the spectral derivative a discontinuity that indicates that an edge of a void formed by the etch process at the region of interest has crossed a layer boundary of the multi-layered structure, determining that the crossed layer boundary corresponds to a preselected layer boundary of the multi-layered structure, and applying a predefined control action to the etch process responsive to determining that the crossed layer boundary corresponds to the preselected layer boundary of the multi-layered structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication No. 62/422,885, filed Nov. 16, 2017, which is incorporatedherein by reference.

BACKGROUND

A critical step in 3D-NAND manufacturing is the high aspect ratio etchthrough multiple layers of a film stack. Controlling the etch process isessential, such as when end point detection is employed to terminate theetch process so as to avoid underetch and overetch.

SUMMARY

In one aspect of the invention a method is provided for controlling anetch process applied to a multi-layered structure, the method includingcalculating a spectral derivative of reflectance of an illuminatedregion of interest of a multi-layered structure during an etch processapplied to the multi-layered structure, identifying in the spectralderivative a discontinuity that indicates that an edge of a void formedby the etch process at the region of interest has crossed a layerboundary of the multi-layered structure, determining that the crossedlayer boundary corresponds to a preselected layer boundary of themulti-layered structure, and applying a predefined control action to theetch process responsive to determining that the crossed layer boundarycorresponds to the preselected layer boundary of the multi-layeredstructure.

In another aspect of the invention the multi-layered structure is asemiconductor.

In another aspect of the invention the void is either of a gate trenchand a channel hole.

In another aspect of the invention the method further includesilluminating the region of interest of the multi-layered structureduring the etch process applied to the multi-layered structure, andmeasuring the reflectance of the illuminated region of interest.

In another aspect of the invention the identifying includes identifyingin the spectral derivative a plurality of discontinuities, where each ofthe discontinuities corresponds to a different layer boundary of themulti-layered structure.

In another aspect of the invention the method further includesdetermining, as any of the discontinuities in the spectral derivativeare identified, a currently etched layer of the multi-layered structurebased on a count of the discontinuities in the spectral derivative.

In another aspect of the invention the method further includesdetermining, as any of the discontinuities in the spectral derivativeare identified, an etch rate based on an elapsed time betweenidentifying any of the discontinuities in the spectral derivative.

In another aspect of the invention the applying includes effectingtermination of the etch process.

In another aspect of the invention the effecting includes effectingtermination of the etch process after a predefined delay.

In another aspect of the invention the predefined delay is based on anetch rate of a currently etched layer of the multi-layered structure.

In another aspect of the invention a system is provided for controllingan etch process applied to a multi-layered structure, the systemincluding an etch layer detector configured to calculate a spectralderivative of reflectance of an illuminated region of interest of amulti-layered structure during an etch process applied to themulti-layered structure, and identify in the spectral derivative adiscontinuity that indicates that an edge of a void formed by the etchprocess at the region of interest has crossed a layer boundary of themulti-layered structure, and an etch process controller configured todetermine that the crossed layer boundary corresponds to a preselectedlayer boundary of the multi-layered structure, and apply a predefinedcontrol action to the etch process responsive to determining that thecrossed layer boundary corresponds to the preselected layer boundary ofthe multi-layered structure.

In another aspect of the invention the multi-layered structure is asemiconductor.

In another aspect of the invention the void is either of a gate trenchand a channel hole.

In another aspect of the invention the system further includes anoptical profile monitor configured to illuminate the region of interestof the multi-layered structure during the etch process applied to themulti-layered structure, and measure the reflectance of the illuminatedregion of interest.

In another aspect of the invention the etch layer detector is configuredto identify in the spectral derivative a plurality of discontinuities,where each of the discontinuities corresponds to a different layerboundary of the multi-layered structure.

In another aspect of the invention the etch layer detector is configuredto determine, as any of the discontinuities in the spectral derivativeare identified, a currently etched layer of the multi-layered structurebased on a count of the discontinuities in the spectral derivative.

In another aspect of the invention the etch layer detector is configuredto determine, as any of the discontinuities in the spectral derivativeare identified, an etch rate based on an elapsed time betweenidentifying any of the discontinuities in the spectral derivative.

In another aspect of the invention the etch process controller isconfigured to apply the predefined control action by effectingtermination of the etch process.

In another aspect of the invention the etch process controller isconfigured to effect termination of the etch process after a predefineddelay.

In another aspect of the invention the predefined delay is based on anetch rate of a currently etched layer of the multi-layered structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the invention will be understood and appreciated more fullyfrom the following detailed description taken in conjunction with theappended drawings in which:

FIG. 1 is a simplified conceptual illustration of a system forcontrolling an etch process applied to a multi-layered structure,constructed and operative in accordance with an embodiment of theinvention;

FIG. 2A is exemplary graph of measured reflectance, useful inunderstanding the invention;

FIG. 2B is exemplary graph of the spectral derivative of measuredreflectance, useful in understanding the invention; and

FIG. 3 is a simplified flowchart illustration of an exemplary method ofoperation of the system of FIG. 1, operative in accordance with anembodiment of the invention.

DETAILED DESCRIPTION

Reference is now made to FIG. 1, which is a simplified conceptualillustration of a system for controlling an etch process applied to amulti-layered structure, constructed and operative in accordance with anembodiment of the invention. In the system of FIG. 1, an optical profilemonitor 100 is configured in accordance with conventional techniques tomonitor a region of interest 102 of a multi-layered structure 104 duringan etch process that is applied in accordance with conventionaltechniques to multi-layered structure 104, such as by etch apparatus106. Multi-layered structure 104 may, for example, be a semiconductorwafer that is being etched to form 3D-NAND flash memory in accordancewith conventional techniques, where multi-layered structure 104 includesa film stack of Oxide-Nitride or Oxide-Poly layers deposited on a basesubstrate, as well as a masking layer. Optical profile monitor 100 isconfigured to illuminate region of interest 102 during the etch process,and particularly during high aspect ratio etch where voids, such as gatetrenches and channel holes, are etched into multi-layered structure 104at region of interest 102. Optical profile monitor 100 is configured toilluminate region of interest 102 and measure the reflectance of theilluminated region in accordance with conventional techniques, such asare described in U.S. Pat. No. 9,528,946.

An exemplary graph of measured reflectance as described hereinabove isshown in FIG. 2A, where reflectance is shown as a function of void depthwhich is used as a proxy for etch time, and where void depth isdetermined in accordance with conventional techniques.

An etch layer detector 108 is configured to calculate a spectralderivative of the reflectance of region of interest 102 as measured byoptical profile monitor 100. Etch layer detector 108 is configured tocalculate the spectral derivative of the reflectance of region ofinterest 102 at different points in time throughout the etch process. Inone embodiment, at each of the different points in time the spectralderivative calculation is of the entire spectrum of the reflected light.Additionally or alternatively, at each of the different points in time adifferent spectral derivative calculation is made at each of a number ofdifferent wavelengths of the reflected light, where any of thesemeasurements or combinations of any of these measurements, such as anaverage thereof, are used as described hereinbelow.

Etch layer detector 108 is configured to identify discontinuities in thespectral derivative, where a discontinuity that indicates that an edgeof one or more voids formed by the etch process at region of interest102 has crossed a layer boundary one of the layers of multi-layeredstructure 104. Etch layer detector 108 is configured to identify aseries of such discontinuities in the spectral derivative, where each ofthe discontinuities corresponds to a different layer boundary ofmulti-layered structure 104.

An exemplary graph of the spectral derivative of measured reflectance asdescribed hereinabove is shown in FIG. 2B, where the spectral derivativeof measured reflectance is also shown as a function of void depth. InFIG. 2B six consecutive discontinuities in the spectral derivative areshown at 200A, 200B, 200C, 200D, 200E, and 200F. Where the number,order, and chemical composition of the layers are known, eachdiscontinuity may be identified as corresponding to a specific layer andits known chemical composition as shown in FIG. 2B.

Etch layer detector 108 is configured to determine, as any of thediscontinuities in the spectral derivative are identified, a currentlyetched layer of multi-layered structure 104 based on a count of thediscontinuities in the spectral derivative. Thus, for example, where thereflectance of region of interest 102 is measured from the start of theetch process and made available to etch layer detector 108, and up to agiven point in time during the etch process etch layer detector 108 hasidentified three discontinuities in the spectral derivative, etch layerdetector 108 determines that the fourth layer of multi-layered structure104 is the currently etched layer.

In one embodiment, etch layer detector 108 is configured to determine,as any of the discontinuities in the spectral derivative are identified,a current etch rate up to a given point in time during the etch processbased on the elapsed time between identifying any, and preferably all,of the discontinuities in the spectral derivative up to that point.

An etch process controller 110 is configured to determine that a crossedlayer boundary as detected by etch layer detector 108 corresponds to apreselected layer boundary of multi-layered structure 104, and apply apredefined control action to the etch process responsive to determiningthat the crossed layer boundary corresponds to the preselected layerboundary of multi-layered structure 104. Thus, for example, if thenumber of layers in multi-layered structure 104 is known prior to thestart of the etch process, such as 48 layers, there are thus 48 layerboundaries not counting the top of the first layer. In this example the48^(th) layer boundary, being the bottom-most layer boundary, may bepreselected such that when etch process controller 110 determines that acrossed layer boundary as detected by etch layer detector 108corresponds to the 48^(th) layer boundary, etch process controller 110applies a predefined control action to the etch process, such as bycausing etch apparatus 106 to terminate the etch process in accordancewith conventional techniques. Alternatively, the 47^(th) layer boundarymay be preselected, and when etch process controller 110 determines thata crossed layer boundary as detected by etch layer detector 108corresponds to the 47^(th) layer boundary, etch process controller 110applies a predefined control action to the etch process after apredefined delay. The predefined delay may be based on the etch rate ofthe currently etched layer of multi-layered structure 104, i.e., the48^(th) layer, such as by using the etch rate to estimate the timeneeded to complete etching the currently etched layer and setting thedelay equal to this amount of time. Additionally or alternatively, thepredefined delay may be based on historical etch rates for layers of thesame or similar chemical composition and geometry.

Reference is now made to FIG. 3, which is a simplified flowchartillustration of an exemplary method of operation of the system of FIG.1, operative in accordance with an embodiment of the invention. In themethod of FIG. 3, a region of interest of a multi-layered structure ismonitored during an etch process that is applied to the multi-layeredstructure (step 300). A spectral derivative of the reflectance of theregion of interest is calculated at different points in time throughoutthe etch process (step 302). Discontinuities in the spectral derivativeare identified, each discontinuity indicating that an edge of one ormore voids formed by the etch process at the region of interest hascrossed a layer boundary of one of the layers of the multi-layeredstructure (step 304). A currently etched layer of the multi-layeredstructure is determined at any point in time during the etch processbased on a current count of the discontinuities in the spectralderivative (step 306). Optionally, a current etch rate of the etchprocess is determined based on the elapsed time between identifying anyof the discontinuities in the spectral derivative (step 308). If acrossed layer boundary corresponds to a preselected layer boundary (step310), a predefined control action is applied to the etch process,immediately or after a predefined delay, such as by effectingtermination of the etch process (step 312).

The flowchart and block diagrams in the drawing figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the drawing figures. For example, two blocks shown insuccession may, in fact, be executed substantially concurrently, or theblocks may sometimes be executed in the reverse order, depending uponthe functionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

The descriptions of the various embodiments of the invention have beenpresented for purposes of illustration, but are not intended to beexhaustive or limited to the embodiments disclosed. Many modificationsand variations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

We claim:
 1. A method of etching applied to a multi-layered structure,the method comprising: measuring of spectral reflectance from a regionof interest of the multi-layered structure during an etch processapplied to the multi-layered structure; calculating a spectralderivative of a spectral reflectance signal obtained from the measuring;identifying, in the spectral derivative, a discontinuity indicative of aof crossing a layer boundary of the multi-layered structure; determiningthat the layer boundary corresponds to a preselected layer boundary ofthe multi-layered structure; and applying a predefined control action tothe etch process responsive to determining that the crossed layerboundary corresponds to the preselected layer boundary of themulti-layered structure.
 2. An etching system for an etch processapplied to a multi-layered structure, the system comprising: etchapparatus; measuring system associated with the etch apparatus andconfigured to measure a spectral reflectance of a region of interest ofthe multi-layered structure during an etch process applied to themulti-layered structure; a control system configured to: calculate aspectral derivative of a spectral reflectance signal obtained from themeasuring; identify, in the spectral derivative, a discontinuityindicative of a of crossing a layer boundary of the multi-layeredstructure; determine that the layer boundary corresponds to apreselected layer boundary of the multi-layered structure; and apply apredefined control action to the etch process responsive to determiningthat the crossed layer boundary corresponds to the preselected layerboundary of the multi-layered structure.